Potassium isotope systematics of oceanic basalts

Abstract High-temperature isotopic fractionation during partial melting and other igneous differentiation processes have been observed in many non-traditional isotope systems. The potassium (K) isotope system has not been extensively investigated historically due to the lack of high-precision analysis methods. The recent development of the Multiple Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS), however, allows for high-precision potassium isotope analysis. In this study, we utilized this new method to analyze 51 geologically, geographically, and geochemically diverse oceanic basalt samples including 32 mid-ocean ridge basalts (MORB), 3 back-arc basin basalts (BABB), and 16 oceanic island basalts (OIB). We observed a limited variation of 41 K/ 39 K ratios across our spread of samples. This variation in mantle-derived rocks is restricted compared to the large K isotopic fractionation observed in low-temperature systems. The averages of MORBs, BABBs, and OIBs are −0.44 ± 0.17‰ (2sd), −0.44 ± 0.08‰, and −0.41 ± 0.16‰, respectively, and there is no geographical variation (e.g., Indian vs. Pacific MORBs) in terms of K isotopes. Among all samples, there are two outliers, in which we have observed evidence of secondary mineral formation (i.e., palagonite) due to interaction with seawater. These two outliers have a K isotopic composition significantly heavier than other unaltered samples, close to the K isotopic composition of seawater. The grand average of all pristine samples is −0.43 ± 0.17‰ (2sd) which agrees well with the Bulk Silicate Earth (BSE) value previously defined. This new study indicates the homogeneity of K isotopes in the mantle and suggests that, since K will not resolvably fractionate during partial melting, any observable fractionation of K isotopes in primitive basalts is likely due to low-temperature, post-eruptive alteration processes. This conclusion is critical for understanding the initial bulk composition of the Earth and it is essential for any interplanetary comparison.